Method of producing finely divided metallic layers



April'4, 1939 N 2,152,809

METHOD OF PRODUCING FINELY DIVIDED METALLIC LAYERS ori inalnifed May 28, 1935 waw.

Patented Apr. 4, 1939 UNITED STATES.

.PATENTX' OFFICE METH D OF rnonucma many DIVIDED METALLIC Lamas I Gerhard Liebmann, Berlin, Germany Original application May 28, 1935, Serial No. 23,845. Divided and this application December 21, 1935, Serial No. 55,633.

May 31, 1934 2 Claims.

ployed more particularly in certain television image recording methods, After corresponding treatment the mosaic screen represents a surface collection of elementary photo-cells, which produce electrical charges corresponding with the particular exposure. further be employed for controlling the transmitter in suitable fashion, by transmitting capacita tively these charges to the silvered rear side con- This appllcanected with an amplifier device.

tion is a division of application Serial Number 23,845 filed May 28, 1935, and now Patent Numiber 2,131,187.

The production of the layer consisting of finest particles of metal is accompanied by considerable difliculties, both as regards selection of the material for the intermediate insulating layer as well as the method of insulation.

- It is possible to employ micafor the intermediate insulating layer. The use of this material when employing a heating process for dividing a metallic layer into single elementary parts is confronted by difliculties owing to the inconstancy of the temperatureand the nature of the surface. When employing a method for producing the distribution by means of engraving machines or in the case of chemical treatment it is impossible in numerous cases, owing to the small 40 corresponding distribution.

According to the invention, there are employed as intermediate dielectriclayerssheets of mica or other suitable bases, forexai'nple, ceramic materiaLcertaln kinds of glass, etc., the surface of this base preferably being made smooth by pole ishing. On this base there is produced in suitpowers of adhesion, to produce a metallic layer in able fashion a metallic coating, for example a silver, copper or the like coating. A metallic coating of this nature may be produced, for example, chemically, by cathode atomization or by thermal application in vapour form. The metallized plates are then heated to high temperature in an atmosphere of indifferent gas or in vacuum. If the metallic coating is of certain thickness (no longer transparent) the temperatures require to These chargesmay then In Germany be very high, for example in the case of silver to about 1000, in order to obtain a brief fusing of the layer of silver, whereby the surface is then converted from the previously unitary structureless metallic surface into avery even mosaic con- 5 sisting of small drops of metal of negligible diameter and insulated against each other. According to the invention, this heating is performed extremely rapidly, preferably by means of eddy current heating in high-frequency furnaces. 10.

In this connection the metallic coating is acted upon not directly, but indirectly by radiation from a heating plate. In the case of very thin metallic layers it is possible to manage with considerably lower temperatures, which in view of VII the low temperature constancy of mica is of ad-. vantage when employing a material of this kind. When using other materials, such as ceramic substances, glass or the like, the surface of the base is preferably made smooth by polishing, in order 20 to accelerate the insular formation during the" heating process.

According to the invention, it is also possible to deposit the metallic layer on a heated base,

so that small globules are likewise formed right 25" from the commencement. When using mica, however, the method of subsequently heating the deposited metal as described in the above is to be preferred.

Anadditional method for insular formation so resides according to the invention in the fact that a thin layer of silver deposited, for example,

on mice. is alternately oxidized and reduced. This method is performed in the most convenient fornrby glow' discharge in an atmosphere of al- 35 ternately introduced oxygen and hydrogen at. sutiable pressure (approximately 1 mm. mercury). The alternate oxidation and reduction might also be replacedto a certain extent by a suitable temperature treatment. In this con- 40 nection the silvered plate is heated in oxygen at first up to the temperature forformation of silver oxide, and then additionally heated in vacuum until the silver, oxide has become dissociated. The temperature is thereupon again low- 45 ered, etc. In the case of 'the glow discharge method the silver layer to be granulated is in the most convenient form placed opposite to a plate connected up as anode, withthe intermediate, connection of' a grid-like electrode connected up 50 as cathode. In this respect the layer of silver does not possess any electrically conductive connection with the cathode. The layer of silver might also be connected up. as cathode. In this,

however, there has been found to exist the dis- 5 Y terial 3, for example mica.

advantage that the oxidizing and reducing process takcs'place more strongly in the vicinity of the connectionbetween the lead and the layer than at other points. Asfinal stage of the process, after production of the discrete particles ofsilver, there is conveniently performed, a light oxidation. as in this manner it is possible to obtain a layer which is advantageously suited for photo sensitization.

In the drawing there is shown diagrammatically by way of example the arrangement for carrying out the method according to the invention. In the drawing- Fig.1 shows the indirect heating by means of a heating plate I, which heats by radiation the layer of silver [mounted on the supporting ma- Since this heating required to commence instantaneously, the heating is preferably performed by eddy currents,

which is represented diagrammatically by the high-frequency coil 4. Fig. 2 shows the arrangement for carrying out the glow discharge process. In the same there is provided an electrode 5, which is connected up as anode and is situated opposite to a grid-like cath- .ode i. The metallic layer 1 to be granulated is arranged within reach of the glow discharge which develops in the case of a suitable filling of gas. 1 is the metallic layer on the base 8. In

Figs. 3 and 4 there is illustrated by way of example the finished product. In these 9 is the intermediate dielectric layer, which acts as base for the coatings l and H. The coating Ill represents the thin layer of metal divided upon into finest elementary particles, whilst II is intended as capacitative counter-coating for the photo-{ cell arrangement.

The experiments according to the invention have proved especially useful the following special embodiment of the present invention.

A well-cleaned mica plate with all grease re;- moved is coated by the thermal vaporization of silver with a silver layer of approximately 95 44 in thickness. A thickness of the layer approximately midway of the stated range, i. e., approximately th to 95 produced the best results. The application of the layer by vaporization is performed in a suitable vessel from which the air has been removed, the distance between the mica plate to be treated and the evaporation furnace being of such extent that a very even coating is obtained. Insulation edges of a desired kind may be allowed to remain by selective screening. After the application of the silver layer by vaporization has been completed a certain amount of air or oxygen is allowed to enter the vessel, and the part of the vessel in which the plate is situated, is heated to approximately 400-450 C. This latter operation extends over a period of approximately 15 to "30 minutes.

becoming entirely useless.

After cooling it will be noticed that the coating on the plate has been divided into single well-sepa rated crystals of about 1;!- in size. 1

It has been found that to obtain this effect it is necessary 1. To adhere to the above stated range as regards the thickness of the layer, and

2. To employ a suitable gas atmosphere.

If the initial layeris made thinner, a granulation ofany kind is not to be determined with the usual optical means available. If the layer is too thick, there is certainly a granulation up to a certain point, but the particles are situated so close together that the requisite insulation maybe not obtained. If on the other hand the layer is still thicker (completely opaque), a distribution according to the described process may not be accomplished at all.

In this case temperatures would require to be employed which come very near to the fusing point of'the silver (800 to l,000 C.), and considerable difliculties are-then encountered-in preventing the mica from The size of the formed particles is also a wholly different one in the last mentionedcase. Further, it has been found that even with suitable layers a separation does not take place when the process is performed in an extremely high vacuum (higher than mm. Hg). The presence of a certain gas atmosphere (presumably oxygen) is accordingly necessary for the separation at the low temperature stated.

Mica plates of this kind, which are furnished with a silver screen in the manner stated, may then be employed in the known fashion, by silvering the rear side and by oxidation and sensitization of the front screen surface, as light-sensitive mosaic electrodes in television transmission tubes, for instance, tubes known under, the name of Iconoscope.

I claim: 7 9

1. In the process of manufacturing a photoelectric mosaic screen for television transmission tubes, the steps of coating a thin mica plate by thermal vaporisation of silver in vacuum with a transparent silver layer of approximately -.5

micron in thickness, and of granulating this layer into fine areas by heating the plate in a rarifled atmosphere containing oxygen to a temperature below 500 C.

2. In the process of manufacturing a photoelectric mosaic screen for television transmission tubes, the steps of coating a thin mica plate by thermal vaporisation of silver in vacuum with a transparent silver layer of approximately -.5 micron in thickness, and of granulating this layer into ilneareas by heating the plate in a gas atmosphere of about 1 mm. mercury pressure during to 30 minutes to a temperature below 500 C.

GERHARD LIEBMANN. 

